1. Field of the Invention
This invention relates to an actuator of a diaphragm type used for actuating, for example, a throttle valve in an automatic cruising control system which controls automatically a speed of an automotive vehicle at a set cruising speed.
2. Description of the Prior Art
Heretofore, there has been used an actuator of a diaphragm type shown in FIGS. 3 and 4 for actuating the throttle valve in the automatic cruising control.
In an actuator 50 shown in FIG. 3, a first casing forming one side of an actuator case is formed with a flange 51a on the outer periphery thereof, and provided with an introduction pipe 5b at the center part thereof. The first casing 51 is provided with a spring fitting part 51c formed in a concave shape surrounding the introduction pipe 51b on the inner side thereof.
A second casing 52 forming another side of the actuator case is provided with a flange engaging part 52a to be engaged with the flange 51a of the first casing 51 on the outer periphery thereof so as to form the actuator case by coupling the first and the second casings 51 and 52 with each other in one body, and formed with a through hole 52b for passing through a diaphragm plate 54 at the center part thereof, which will be described later.
A diaphragm 53, which is made of rubber, is formed with a rim 53b on the outer periphery thereof, and provided with a plate fitting part 53c formed in annular shape in the center part thereof, as shown also in FIG. 4.
The diaphragm 53 is fixed in the actuator case by holding the rim 53b between the flange 51a of the first casing 51 and the flange engaging part 52a of the second casing 52, and attached with the diaphragm plate 54 in the plate fitting part 53c thereof.
The diaphragm plate 54 is formed with a cable connector 54a engaged with a throttle cable 60 connected to a throttle valve (not shown) of the vehicular engine, a recess 54b for engaging with the plate fitting part 53c of the diaphragm 53 and a projection 54c for fitting a conical spring 55. The diaphragm plate 54 is attached to the diaphragm 53 by fitting the plate fitting part 53c of the diaphragm 53 into the recess 54b of the diaphragm plate 54 according to the function of the elasticity of the diaphragm 53.
The conical spring 55 is disposed between the diaphragm plate 54 and the first casing 51 of the actuator case by engaging its both ends to the projection 54c of the diaphragm plate 54 and the spring fitting part 51c of the first casing 51. Thus, a negative pressure chamber 56 is formed between the first casing 51 and the diaphragm 53 attached with the diaphragm plate 54 energized by the conical spring 55 in the leftward direction in FIG. 3.
In the actuator 50 having the aforementioned structure, when negative pressure is introduced into the negative pressure chamber 56 through the introduction pipe 51b according to a command output from a controller of the automatic cruising control system (not shown), the pressure in the negative pressure chamber 56 is reduced and the diaphragm plate 54 moves together with the diaphragm 53 against the elastic force of the conical spring 55 in the rightward direction in FIG. 3.
According to the rightward movement of the diaphragm plate 54, the throttle valve of the vehicular engine is actuated in the opening direction through the throttle cable 60, thereby accelerating the vehicle so as to maintain the vehicle speed constantly at the set cruising speed.
Then, when atmospheric pressure is introduced into the negative pressure chamber 56 through the introduction pipe 51b by operating valves (not shown) according to a command output from the controller, the diaphragm plate 54 moves together with the diaphragm 53 by the elastic force of the conical spring 55 in the leftward direction in FIG. 3, and the throttle valve is returned in the closing direction through the throttle cable 60.
In this time, the plate fitting part 53c of the diaphragm 53 is fitted in the recess 54b of the diaphragm plate 54 so as not to leak the pressure in the negative pressure chamber 56.
However, the aforementioned conventional actuator 50 is so constructed that the diaphragm plate 54 is attached to the diaphragm 53 by fitting the plate fitting part 53c of the diaphragm 53 into the recess 54b of the diaphragm plate 54, that is merely by the function of the elastic force of the diaphragm 53, consequently, there is the possibility that the plate fitting part 53c of the diaphragm 53 is pulled outwardly and the plate fitting part 53c is disengaged from the recess 54b of the diaphragm plate 54 if the pressure in the negative pressure chamber 56 is reduced excessively. Accordingly, there is a problem in that it becomes impossible to actuate the throttle valve of the vehicular engine and control the vehicle speed constantly at the set cruising speed since the air flows into the negative pressure chamber 56 in the case where the plate fitting part 53c of the diaphragm 53 is disengaged from the recess 54b of the diaphragm plate 54.